BR should provide the total size of one backup considering following cases:

• Users want to know the size of each backup.
• The platforms that integrate with BR, e.g. TEP, DBaaS, etc. need to show the size of each backup.

Maybe TiKV can return the size of the files it created for each backup to BR and BR calculates the total size from all the TiKVs and writes the size to backup meta.

@kennytm @overvenus @tennix